In an electric power steering device in which an output of an electric motor is transmitted to a steering turning mechanism through a reduction gear mechanism, a worm reduction gear mechanism may be sometimes used as the reduction gear mechanism (for instance, see Patent Document 1).
In a structure disclosed in Patent Document 1, an output shaft of the electric motor is connected to a worm shaft by a spline joint. Specifically, a female spline formed in the output shaft of the electric motor is fitted to a male spline formed in one end portion of the worm shaft.